1. Selection of mutants lacking orotidine-monophosphate-pyrophosphorylase (ura5) and orotidine-5'-phosphate decarboxylase (ura3) from the two varieties of Cryptococcus neoformans; 2. Physical mapping of C. neoformans genes; 3. Molecular identification of C neoformans var. gattii isolated from Eucalyptus camaldulensis in Australia and San Francisco; 4. Stress induced chromosomal rearrangement in Candida albicans. Spontaneous mutants requiring uracil were isolated from both varieties of C. neoformans by plating on 5-fluoroorotic acid (5-FOA) medium. Of the 36 strains (18 each from the two varieties), 24 generated (12 each) resistant cells requiring uracil for growth. The uracil requiring mutants of C neoformans var. gattii were identified either as ura3 or ura5 in equal frequency. The ura3 isolates were identified by enzyme assay while ura5 was identified by genetic complementation with a URA5 gene cloned from C. neoformans var. neoformans. The uracil requiring mutants of var. neoformans, however, were almost always ura5. A total of 10 isolated genes were used as probes to construct a linkage map by hybridizing the probes to the chromosomes separated by pulsed-field gel electrophoresis. The linkage map of A and D serotypes of C neoformans were nearly identical but differed from that of C. neoformans var. gattii. The ecological niche of C. neoformans var. gattii remained an enigma until 1990 when it was isolated from Eucalyptus camaldulensis (red river gum tree) in Australia as well as California. The identity of isolates from the trees were confirmed by using a C neoformans ribosomal DNA probe (Gen-probe) and electrophoretic karyotype.